1. Field of the Invention
The present invention relates to an apparatus and method for acquiring synchronization using periodically repeated patterns of a preamble.
2. Description of the Related Art
In mobile communication systems, a transmitter transmits a synchronization signal to a receiver and the receiver acquires synchronization using the received synchronization signal. As one of such communication systems, the standardization committee for the IEEE 802.16 standard has recently suggested a communication system adopting an Orthogonal Frequency Division Multiple Access (OFDMA) technique for high-speed data transmission. According to the IEEE 802.16 standard, in an OFDMA communication system, a transmitter transmits a preamble pattern to a receiver and the receiver acquires the start point of a frame, i.e., frame synchronization, using the received preamble pattern.
FIG. 1 illustrates a preamble pattern used for initial synchronization in a communication system. A preamble 10 includes repeated patterns 12, 13, and 14 and a cyclic prefix (CP) 11. In OFDMA communication systems, symbol transmission is performed on a symbol-by-symbol basis, but an Orthogonal Frequency Division Multiplexed (OFDM) symbol is affected by previous OFDM symbols during transmission through a multipath channel. To avoid such interference between OFDM symbols, a guard interval that is longer than a maximum delay spread of the multipath channel is inserted between consecutive OFDM symbols. To prevent the destruction of orthogonality which may occur due to the delay of subcarriers, the last part of a valid symbol interval is copied and inserted into the guard interval, which is called a cyclic prefix.
A technique for estimating the start of a frame has been disclosed in U.S. Pat. No. 6,459,745, (the '745 patent) entitled “Frequency/Timing Recovery for Orthogonal Frequency Division Multiplexed Signals.” In the '745 patent, based on the structure of a preamble that includes repeated patterns of a signal having a length L as shown in FIG. 1, a frame synchronization algorithm estimates the start of a frame using a given circumstance that a correlation between the repeated patterns is maximal. In other words, the '745 patent discloses calculating a correlation between a signal received during a time window of a predetermined size and a predetermined delay of the received signal and searching for the start point of a frame using the calculated correlation. For example, when the window size of a preamble is W and the predetermined delay is D (not shown), the structure of an apparatus for estimating the start of a frame is as shown in FIG. 2. In FIG. 2, W and D can be selected from among several values according to the structure of a preamble and the way of implementing the preamble. For example, when a preamble includes 3 repeated patterns, W may be equal to 2L+CP and D may be equal to L.
FIG. 2 is a block diagram illustrating a conventional apparatus for estimating the start of a frame. The apparatus for estimating the start of a frame includes a conjugator 22, a first delayer 24, a correlator 26, a second delayer 28, a summer 30, and a third delayer 32. The conjugator 22 calculates the conjugate of a received signal and outputs the conjugate to the correlator 26. The first delayer 24 delays a received signal by a repetition interval of L and outputs the delayed signal to the correlator 26. The correlator 26 calculates a correlation between the received signal and the delayed signal and outputs the correlation to the summer 30 and to the second delayer 28. The second delayer 28 delays the correlation output from the correlator 26 by a window size W of a preamble and outputs the delayed correlation to the summer 30. The summer 30 sums the correlation output from the correlator 26 and the delayed correlation output from the second delayer 28. The third delayer 32 delays a value output from the summer 30 by 1 and outputs the result to the summer 30. Since the output of the second delayer 28 is subtracted from a value resulting from operations of the third delayer 32 and the summer 30 that continuously sum outputs of the correlator 26, the second delayer 28, the third delayer 32 and the summer 30 function together to sum up the outputs of the correlator 26 only during a W of a preamble.
The correlation is determined by Equation 1 as follows.
                              C          ⁡                      (            n            )                          =                              ∑                          k              =              0                                      N              -              1                                ⁢                                    r              ⁡                              (                                  n                  +                  k                                )                                      ·                                          r                *                            ⁡                              (                                  n                  +                                      k                    ÷                    D                                                  )                                                                        Equation        ⁢                                  ⁢        1            
When n0 is the start point of a frame, C(n) has the maximum value (or peak value) at n=n0. Thus, the start point of a preamble, i.e., the start point of the frame, can be estimated by searching for a position in which |C(n)| is maximal within a predetermined interval of n.
Such a conventional technique is based on the assumption that a preamble is comprised of repeated patterns of a signal having a length of L. This assumption cannot be satisfied when a high-level interference signal is input. To create a preamble having repeated patterns in a multi-carrier system such as an OFDM system, a 0 should be periodically inserted between transmission signals in a frequency domain and the transmission signals should be converted into time-domain signals through inverse fast Fourier transform (IFFT) for transmission. For example, in the case of a preamble suggested in an IEEE 802.16e communication system, a signal is inserted into only one of three subcarriers, and the other two subcarriers are each filled with a “0” and then undergo an IFFT. Thus, the preamble includes three repeated patterns.
However, when a preamble having such a structure is created in a multi-sector environment, a position in which a signal is to be inserted varies from cell to cell, thereby preventing performance degradation caused by interference in sector ID estimation or channel estimation using a preamble.
FIG. 3 is a graph illustrating the use of frequencies and their respective sectors in a multi-sector environment in a wireless communication system. In a wireless communication system, a cell can be divided into a plurality of sectors. Each sector has three repeated patterns by inserting a signal into only one of three subcarriers and filling the other two subcarriers with 0s and performing IFFT on the two subcarriers in a frequency domain. However, in the multi-sector environment, signals of sectors are mixed at the boundary between the sectors. Therefore, a received signal may actually include signals from more than the sector and may even contain signals from all three sectors, thus, increasing the likelihood of receiving a signal other than a 0. As a result, as shown in the embodiment of FIG. 3, frequency-domain signals having no 0s are filled in all the subcarriers, causing weakening of a repeated pattern of a time-domain signal. Thus, conventional techniques based on the repeated pattern of the time-domain signal experience severe performance degradation.